Thermography is an imaging technique based on infrared emission by an object at a particular temperature (grey body radiation). Thermography may include passive thermography or active thermography. Active thermography involves applying a stimulus to a target to cause the target to heat or cool in such a way as to allow characteristics of the target to be observed when viewed by thermal imagery. Active thermography plays a crucial role as a non-destructive technique (NDT) in many industries, especially in aerospace. Electromagnetic excitation is the most commonly used way of exciting the sample among thermographic techniques.
Thermography may also be classified as point imaging thermography, line imaging thermography, area imaging thermography and three-dimensional (3D) imaging thermography (tomography). Area or 3D imaging thermography may also be classified as one-sided or two-sided thermography. Active area imaging thermography may include a detector such as an infrared camera, a heating source as well as image processing software. For example, in a conventional one-sided optically excited thermography system, a flash lamp may be used as a source of electromagnetic radiation to illuminate a surface of a sample. An infrared camera is used to record the temperature evolution of the sample surface. The source and the detector are arranged on the same side in relation to the sample in the one-sided optically excited thermography. The camera and the flash lamp may be coupled to a computer. The computer may be configured to acquire data from the camera as well as configured to control the camera and flash lamp.
As another example, in a conventional two-sided optically excited thermography system, an infrared radiator or heating lamp may be used as a source of electromagnetic radiation to illuminate a sample. The infrared camera is arranged at the side (of the sample) opposite the radiator to record the temperature evolution. The source and the detector may be arranged on opposite sides in relation to the sample in the two-sided optically excited thermography. The camera may also be coupled to a computer.
The lamp radiator may be a tungsten filament lamp with broad spectral response but in general, alternative sources with spectral components from UV to microwave may also be utilized. This technique has been particularly successful in finding delaminations in Fiber Reinforced Plastics (FRP). The thermo-physical properties of such defects display a high contrast to the fibers and matrix of FRP. Such substantial contrasts allow the lateral conduction in the FRP to be disregarded and heat propagation within the sample be treated as a one-dimensional (1D) problem, making it possible to extract depth information from the thermography data. However, 1D model may only be valid if 3D diffusion can be ignored. For this to happen, one or several of the following criteria should be satisfied:                The surface heating is uniform, so that there are no lateral gradients.        The contrast in thermo-physical parameters between defect and sound regions of the sample is high enough to create temperature gradients much larger in comparison with deviation from one dimensional (1D) solution.        The detection is performed shortly after the heat source is switched off, so that heat diffusion is minimal. Similarly this criterion can be defined if the location of the defect is close to the surface.        
Another conventional approach is based on laser heating. In this approach, the heating may be performed in a non-uniform manner. Through such non-uniform heating, it is possible to detect defects that strongly affect lateral heat flow, like cracks. One of the recent examples is the flying laser spot thermography system. The interaction of laser with the surface is monitored continuously using an IR camera. When the laser spot is in the vicinity of a crack, the higher thermal resistivity of the crack leads to a reduced cooling and thus to a higher maximal temperature. Eventually, it gives rise to the thermal crack signature. By differentiation of the temperature profiles in different direction, the crack orientation can be reconstructed.
Few thermography methods based on laser are also employed for material properties evaluation. For example, in Time Resolved Infrared Radiometry (TRIR), the heating with a laser is used to determine thickness of the coating or the presence of delaminations. The same TRIR set-up may be used for detection of delaminations under the coating, which behave as disbonded material.
In the field of aerospace, different techniques have been applied for cracks and delaminations detection. Not restricting to active thermography, these techniques include x-ray examination, dye-penetrant, ultrasound, eddy-currents, etc., and discussion on the disadvantages has been carried out. Different methods in different set-up and orientation specifically targeting thermal imaging were also discussed.
A more recent discussion in the NDT industry involves the possibility of manipulating the shape of the source intensity with a constant output which clearly relates to forced diffusion thermographic instrument. Similar concept was also disclosed which uses the line-scanning method to heat and measure the sample with a photothermal test camera while the system design also allows manipulation of the laser beam shape. The contribution of ideas over the years prompted the introduction of algorithm calculation to enhance the capability of such systems. However, they only mention about the detection of high contrast defects such as cracks and delaminations, but none was found to include the detection of low-contrast defect such as minor heat damage.
There are several limitations of the conventional active thermography based on flash lamps, which limit its application only to the defects with high contrast in thermophysical parameters in relation to base material under inspection:                It is too challenging to achieve uniform illumination of the material, which introduces lateral temperature gradients that will dominate the IR image.        Even if the uniformity of illumination can be achieved, it is practically impossible to avoid variation of the light absorption at the surface, which will depend on material composition, surface structure and finishing and presence of surface contamination.        Even after the flash is applied, the glow from the lamp stays strong for several seconds and is reflected from the sample into the camera. This makes it impossible to use the thermography at early stages of thermal transition.        Application in ambient condition causes cooling of the surface through convection, which contributes significantly after 10 seconds of observation.        
Some of the limitations of the thermography based on single laser are listed below:                It requires scanning of the single beam, which restricts the analysis to a relatively small area of the sample that can be examined in a reasonably short time.        Single laser approach makes it difficult to compare two different spots within the area of interest. The reason for this is that the analysis of one spot inevitably leads to the temperature increase in the whole part under investigation. Hence, the initial temperature condition for each consecutive spot is different. This issue can be ignored for high-contrast defects, like disbonds and cracks, but it will be detrimental for low-contrast defects, like incipient heat damage.        
A need therefore exists to provide a method of detecting defects in an object based on active thermography and a system thereof, which seek to overcome, or at least ameliorate, one or more of the deficiencies of the conventional art mentioned above. It is against this background that the present invention has been developed.